Stop a liberal being trite (5)
I believe the answer is:
banal
'trite' is the definition.
(I've seen this before)
'stop a liberal' is the wordplay.
'stop' becomes 'ban' (I've seen this before).
'liberal' becomes 'l' (abbreviation).
'ban'+'a'+'l'='BANAL'
'being' is the link.
(Other definitions for banal that I've seen before include "Tediously commonplace" , "flat" , "Stock" , "Uninteresting, commonplace" , "Obvious and dull" .)